In cells of semiconductor devices such as a DRAM (Dynamic Random Access Memory), device miniaturization has led to the need to form fine contact plugs. Patent literature article 1 discloses a method of forming two adjacent fine contact plugs (referred to as twin plugs hereinafter).
In the method of forming twin plugs disclosed in patent literature article 1, first a large space corresponding to the size of two contacts is opened between wiring lines, and is filled with a plug material such as doped polysilicon. The actual plug portions are then masked and are separated by etching, and the separating portion is filled with an insulating film (referred to as an isolating and insulating film).